Tetrahydrobiopterin (BH4) is a vital cofactor that maintains availability of amine neurotransmitters including Dopamine and Serotonin, regulates Nitric Oxide synthesis, and stimulates and modulates the Glutamatergic system. Dysregulation of neurotransmitter (NT) systems has been implicated in the pathogenesis of schizophrenia (SZ). Based on the central roles of BH4 in NT synthesis, we assayed plasma total biopterin levels (a measure of BH4) in a large sample of patients with SZ and control subjects. A highly significant mean biopterin deficit of 34% was observed for the SZ patient group when compared to controls. The observed biopterin deficit is comparable to that reported for genetic BH4 deficiency disorders, supporting its characterization as having physiological significance. Our highly significant finding, along with: a) the known roles of BH4 in NT maintenance, b) dysregulation of CNS NT synthesis observed in human and mouse BH4 deficiencies, c) evidence that plasma biopterin levels are correlated with CNS biopterin levels, and d) evidence that urinary biopterin excretion is not increased in SZ, all support our hypothesis that dysregulation of BH4 biosynthesis is involved in the etiology of SZ. SZ susceptibility has a large genetic component, and in another preliminary study, we have performed initial genetic testing of GCH1, the first gene in the BH4 biosynthesis pathway, genotyping a polymorphism for association with SZ in a sample of 132 subjects (86 SZ and 46 control subjects). The results were striking: We found that GCH1 was strongly associated with SZ: odds ratio, 5.0, p= 0.0057. We also found that GCH1 allele status predicts low biopterin in SZ patients. Based on these very positive preliminary data, and the fact that some SZ subjects without the SZ-associated GCH1 genotype also have a biopterin deficit, this exploratory study is designed to test the hypothesis that DNA variants in GCH1 and other BH4 biosynthesis-related genes, are associated with and influence biopterin levels, and are candidate SZ susceptibility loci. We will employ Re- Sequencing Arrays to screen for DNA variants that may influence gene expression and/or protein function. Sequence data generated in this study will be analyzed for association with low biopterin levels and subject diagnosis. The Specific aims of this proposed study are: Aim 1a. To design Re-Sequencing arrays that include the gene regulatory regions, intron-exon boundaries, and coding regions for the six BH4 biosynthesis pathway genes, plus 13 additional BH4 regulation-associated genes. 1b. To generate gene sequence data for 180 subjects (90 SZ subjects and 90 controls), all with assayed plasma biopterin levels. Aim 2. To analyze the sequence data using single marker association testing and haplotype analyses, and to identify DNA variants associated with SZ and biopterin deficit. Understanding the genetic basis of the SZ biopterin deficit will be primary to development of medications and may also allow pre-symptomatic and early diagnostic testing, and early treatment that may improve the clinical outcomes for persons at risk of developing SZ. 7. PROJECT NARRATIVE This study will test whether people with schizophrenia (SZ), who showed abnormally low levels of a chemical called BH4 that is very important for brain function, also have DNA abnormalities in the genes that produce BH4. If they do, we will look for ways to treat low BH4 levels, and try to improve the symptoms of patients that way. We may also be able to predict risk of SZ in people with the gene defect, and therefore may be able to start treatment as early as possible to improve the outcome for those with the disease and young people at risk of developing SZ. [unreadable] [unreadable] [unreadable]